1. Field of the Invention
This invention relates to a sintered ceramic body for use in cutting tools. More specifically, this invention relates to a sintered ceramic body of a novel composition having excellent thermal shock resistance. A method for producing such a sintered body is also provided.
2. Discussion of the Prior Art
A sintered body comprising alumina (Al.sub.2 O.sub.3) as a main ingredient is the only sintered ceramic body now available for producing cutting tools which can be used at a cutting speed of more than 200 m/min. However, it is susceptible to breakage in milling, etc. Various attempts have therefore been made to increase its strength by adding a carbide such as TiC, WC or Mo.sub.2 C, a nitride or nitridecarbide such as TiN, ZrN or TiCN, or a boride such as TiB.sub.2 to the Al.sub.2 O.sub.3. However, such ceramics have low resistance to thermal shock, and are suspectible to breakage in milling, etc. These defects are attributed to this low thermal conductivity. Among the aforesaid additives for Al.sub.2 O.sub.3, WC has a thermal conductivity of 0.45 cal/cm.sec..degree.C., the highest among them. Addition of WC, therefore, ought to increase the thermal conductivity of the sintered ceramic body and, therefore, its thermal shock resistance. Actually, however, the result is much the same as in the case of adding TiC which has a thermal conductivity of 0.04 cal/cm.sec..degree.C.
Extensive investigations of Al.sub.2 O.sub.3 -WC ceramic bodies have led to the discovery that because (i) fine pores with a size of less than 0.5.mu. are liable to occur in such a sintered ceramic body, (ii) a part of the carbon in WC reacts with Al.sub.2 O.sub.3 to form brittle W.sub.2 C in the crystals, and (iii) the grain growth of Al.sub.2 O.sub.3 is vigorous; a decrease in strength occurs, thermal shock resistance is not enhanced, and the result is much the same as in the case of using other additives having a lower thermal conductivity.
Further investigations have led to the discovery that when a certain specified amount of TiN is added together with WC to Al.sub.2 O.sub.3, a sintered ceramic body can be obtained which is free from deterimental pores and has a crystal grain size of not more than 2.mu. and excellent thermal shock resistance. When a cutting tip is made from this ceramic and used in milling, it shows more than twice as high a breaking resistance as conventional tools of the Al.sub.2 O.sub.3 -carbide (nitride) type.